Microscopes are essentially devices which create maps or displays of the variation of some property of an object under study. Different types of microscopes map variations of different properties of a material to provide contrast in a generated image of the material.
Optical microscopy techniques can be used to generate maps of the variations of some properties of certain materials. However, diffraction effects and depth of field limitations present formidable difficulties when attempting to discern variations in the properties of materials in which the variations are on a scale of the order of 5-10 angstroms. Optical microscopes use light with wavelengths of the order of a few thousand angstroms; the resolution of optical microscopes, it should be noted, is at best about 2500 angstroms.
Electron microscopy techniques have been used in the mapping of very fine variations in topography of certain materials. However, the electron microscope, even though overcoming the diffraction and depth of field difficulties experienced by optical microscopes, nevertheless, is limited by its field of view. Moreover, electron microscopes have another limitation in that the preparation of a specimen of a material to be evaluated typically requires cutting an area or portion of interest out of the material in order to provide a specimen small enough to fit inside the vacuum chamber of the electron microscope.
Acoustic microscopy techniques are used to determine absorption spectra and the Raman frequency modes of material. See U.S. Pat. Nos. 4,028,933 and 4,267,732 for a detailed description of these techniques. Acoustic microscopes can be used to discern topographic, mechanical and thermal properties of a material. Acoustic microscopes, however, cannot discern electrical properties of materials.
Furthermore, optical, electron and acoustical microscopes provide a common difficulty in that they can present a great number of extraneous features which are not relevant to certain kinds of material evaluation.
In the art of video disc records, and the manufacture of the discs useful in the art, it is important to be able to determine certain properties of the disc.
It is known that the video disc that has been recorded with information comprising both video and audio signals still contains in the playback mode extraneous signals which are termed noise. These noise signals contribute deleteriously to the quality of the video and audio signals that are eventually displayed in a TV-monitor. Variations in the (1) geometry of the groove or what may be termed variations from the desired topography, (2) the mechanical stiffness of the groove, or more particularly, the mechanical stiffness of the surface layer of the material and (3) the complex dielectric of the material of the disc all contribute to the noise signals. While it is desirable that these properties be identified, no known process heretofore has been able to provide such information. The optical, acoustic, and electron microscopes can, in principle, discern variations in the geometry. However, in practice, optical and acoustic microscopy of the variation of the groove geometry of concern in the video disc art is impossible because the variations of interest are more than 1,000 times smaller than the groove itself and because the dimensions of the variation of interest are at or beyond the limits of optical microscopes.
Electron microscopes can discern the variations of geometry which are of interest, but only over such a small field of view as to make the interpretation of a display from an electron microscope very difficult. Electron microscopes also probe fairly deeply into the surface of the material of a disc, thereby further complicating interpretation of the displays, which are usually in microphotograph (or, simply, "micrograph") form. There is a need, therefore, for a system that functions as a microscope to provide detailed and enlarged mapping displays manifesting or representing the variations in the properties of the surface of materials.